LINQ error

I have a Folder table which has a one-to-many association with a Menu. I have generated entities for these tables in the designer.

When I run this LINQ query:

return UnitOfWorkScope.Current.Folders.Where(f => f.Menu.Name + "-" + f.SerialNumber == reference.Trim()).SingleOrDefault();

I get the following error:

Mindscape.LightSpeed.LightSpeedException : Query Error: Could not find field [SerialNumber] on model [Menu]
    at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)
    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
    at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
    at Mindscape.LightSpeed.Linq.ProjectionStrategy.ExecuteEntityQuery(Query query, IUnitOfWork unitOfWork)
    at Mindscape.LightSpeed.Linq.ProjectionStrategy.Execute(Query query, IUnitOfWork unitOfWork)
    at Mindscape.LightSpeed.Linq.LinqQueryExecutionPlan.Execute()
    at Mindscape.LightSpeed.Linq.Sqo.SingleBase.Evaluate(LinqQueryExecutionPlan plan)
    at Mindscape.LightSpeed.Linq.Sqo.ImmediateSqoWithPredicate.OnVisit(LinqQueryTranslator visitor, MethodCallExpression expression)
    at Mindscape.LightSpeed.Linq.Sqo.SqoBase.PerformVisit(LinqQueryTranslator visitor, MethodCallExpression expression)
    at Mindscape.LightSpeed.Linq.ExpressionVisitor.Visit(Expression exp)
    at Mindscape.LightSpeed.Linq.LinqQueryTranslator.Translate(Expression expression, IUnitOfWork unitOfWork)
    at Mindscape.LightSpeed.Linq.LinqQueryProvider.System.Linq.IQueryProvider.Execute(Expression expression)
    at System.Linq.Queryable.SingleOrDefault[TSource](IQueryable`1 source)

The error message is misleading -- what this is actually trying to tell you is that you can't use attributes from multiple entity types in the same query.  Your query combines a Menu (for its Name attribute) and a Folder (for its SerialNumber attribute).  (See Help Topics > LINQ Support > Limitations of the LINQ Provider in the user guide, though this is actually a limitation of the LightSpeed core query engine.)